What causes flies to appear in your home?

One of the most common reasons why flies would venture into your home is if...
1. You have food
2. By coincidence through a large opening, such as a door

Another reason is that the flies are attracted to light, so at night, if your door is open, they could go inside your house. Also, avoid using any type of perfume or products like it inside your house, or otherwise you would have to close all openings. I hope this was helpful.